1. Field of the Invention
The present invention relates to a semiconductor optical amplifier device and to a semiconductor optical amplifier driving apparatus. More specifically, the present invention relates to a semiconductor optical amplifier device amplifying an external light signal and to a driving apparatus therefor.
2. Description of the Background Art
Along with penetration of large capacity communication, all-optic processing, in which light signals are processed as they are at high speed without converting to electricity, has been promoted. One of the most important tasks to be accomplished regarding the light signals is to increase speed while maintaining signal quality.
In current optical communication systems, a light signal obtained by intensity-modulation of an output light of semiconductor laser is transmitted through an optical fiber. The light signal is reflected time and again at the inner wall of the fiber as it proceeds through the fiber and, at every reflection, the signal intensity lowers because of absorption loss and scattering loss. As a result, the light signal waveform deforms, signal-to-noise ratio (hereinafter denoted as S/N ratio) indicating signal quality lowers, and signal transmission quality degrades.
Various factors deteriorate the light signal as it passes through the optical fiber and through many devices such as repeaters and switches. Influence of waveform deterioration and degradation of the light signal experienced on the route grows as the signal speed increases. Degradation of light signal is a serious cause of increased bit error rate (BER) and it significantly lowers signal quality.
Therefore, it is important for an optical communication device, which processes repetitive pulse light signals of ultra-high speed essential to large capacity optical communication, to have good response characteristic and to curb attenuation along the transmission path.
A high-frequency signal is much affected by “pattern effect” that degrades response characteristic, and hence, it is a significant factor determining signal quality. Dependent on whether a pulse exists at a signal position or not, density of injection carriers remaining in an active layer of a laser device varies at the next signal position, and the pattern effect occurs due to this variation of injection carrier density. Whether there is a pulse at a signal position or not corresponds to “1” and “0” of a binary signal.
With the carrier life of electrons being several ns, a high-frequency signal of which period is comparable to or longer than the carrier life would be influenced if the preceding signal is “1”, as the residual carriers exist at the next signal position. Particularly, when the semiconductor laser is provided with a saturable absorption region, the influence of residual carriers increases, as the carrier life is long in the saturable absorption region. Because of the pattern effect as such, signal waveform may be lost as the bias is effectively applied at the next signal timing or the fall of the signal trails so long as to overlap the next signal. In that case, even at the position of a signal “0”, the light is not off and, hence, coefficient of extinction degrades.
In view of the foregoing, a receiver having good response characteristic capable of preventing increase in bit error rate and detecting a deteriorated weak signal has been developed.
At present, a photodiode (PD) using an InGaAs-based semiconductor material for long wavelength range has been widely used in receivers for optical communication. By enlarging light receiving area of the photodiode, sensitivity of the receiver can be improved. When the light receiving area of the photodiode is increased, however, speed of response lowers. Therefore, it is difficult to use the photodiode for large-capacity and ultra-high-speed future optical communication.
Recently, a new photodiode called Uni-Traveling-Carrier Photodiode (UTC-PD) has been proposed, in Japanese Patent Laying-Open No. 09-275224. In a common photodiode, carriers that travel include both holes and electrons. The holes travel slow, and prevent fast response. Therefore, in the UTC-PD, holes are not used and only the electrons that travel fast are used, to enable high-speed response of several hundreds GHz or higher.
As is naturally understood from the principle of UTC-PD that fast response and high output property are realized by using only the electrons as traveling carriers, the performance of sensing weak signals is substantially the same as that of the conventional photodiode. Therefore, UTC-PD is not suitable for detecting a light signal deteriorated by transmission loss.
Technique for solving transmission loss by amplifying and waveform-shaping a deteriorated light signal has been developed. As an example, reproduction of light signal utilizing bistable semiconductor laser having a saturable absorption region will be described with reference to the figures.
FIG. 14 is a graph showing injection light-optical output characteristic curve of a general bistable semiconductor laser having saturable absorption region. FIG. 15(a)-15(c) illustrate operational characteristics of a conventional light signal reproducing apparatus using the bistable semiconductor laser having a saturable absorption region.
FIG. 15(a) represents the injection light-optical output characteristic curve of the bistable semiconductor laser shown in FIG. 14; FIG. 15(b) represents a light signal PIN injected to the bistable semiconductor laser having the characteristic of FIG. 15(a); and FIG. 15(c) represents output light Pout obtained by injecting the light signal PIN of FIG. 15(b) to the bistable semiconductor laser having the characteristic of FIG. 15(a).
When a light signal that is modulated between an optical intensity below a threshold value Pth2 and an optical intensity above an oscillation threshold value Pth1 is injected as shown in FIG. 15(b) to the bistable semiconductor laser having the injection light-optical output characteristic as shown in FIG. 15(a), an amplified optical output can be obtained from the bistable semiconductor laser as shown in FIG. 15(c). Thus, effects of improved S/N (Signal to Noise) ratio and waveform shaping can be attained.
The bistable semiconductor laser as described above is capable of amplifying and detecting a weak light signal of the level around oscillation threshold value. The performance, however, is limited as the light signal cannot be detected unless the intensity of injected light goes up or down across the threshold value. Therefore, it cannot be used for a light signal having small amplitude or attenuated below the threshold.
Japanese Patent Laying-Open No. 2004-214407 discloses, as another example that can compensate for the transmission loss, a stochastic resonance apparatus performing amplification and waveform shaping of an input signal. For the stochastic resonance apparatus requiring non-linear input-output characteristic, a bistable semiconductor laser 112 of FIG. 16, for example, having non-linear characteristic, is used.
FIG. 16 is a cross-sectional view showing a structure of a general bistable semiconductor laser 112 used as the stochastic resonance apparatus.
Referring to FIG. 16, bistable semiconductor laser 112 includes: an active layer 120 including an optical amplifying region 121 and a saturablel absorption region 122; p-type electrodes 123 and 124; a p-type clad layer 125; an n-type clad layer 126; and an n-type electrode 127. On the n-type electrode 127, n-type clad layer 126 is provided. On the n-type clad layer 126, active layer 120 is provided. On active layer 120, p-type clad layer 125 is provided. On p-type clad layer 125, p-type electrodes 123 and 124 are provided.
Bistable semiconductor laser 112 receives input light PIN from optical amplification region 121 and outputs output light POUT from saturable absorption region 122. The stochastic resonance apparatus such as shown in FIG. 16 inputs a signal, obtained by adding noise to a signal that has been deteriorated along the transmission path to be too weak to exceed hysteresis threshold value of the bistable semiconductor laser, to the bistable semiconductor laser. At this time, the output signal goes up/down across the hysteresis corresponding to the peak of the input signal and, therefore, an output signal of which waveform is well shaped and intensity amplified to emphasize the input signal period can be obtained.
By the above-described effect of stochastic resonance, it becomes possible to detect, amplify or shape the waveform of such a weak signal that cannot be detected by a common functional element.
Japanese Patent Laying-Open No. 02-137383 discloses, as another example that can obtain a signal with magnified amplitude, a driving method in which current is injected to the saturable absorption region in the semiconductor laser device in a bistable state to apply a bias voltage in the forward direction (hereinafter referred to as forward bias voltage).
FIG. 17 is a cross-sectional view showing a schematic device structure of a conventional semiconductor laser device 160 of the bistable state.
Referring to FIG. 17, semiconductor laser device 160 includes an optical amplification region 161, a saturable absorption region 162, and an active layer 163. Semiconductor laser device 160 is formed as an AlGaAs/GaAs lateral mode control type semiconductor laser device formed on a GaAs substrate. As shown in FIG. 17, in semiconductor laser device 160, electrode on one side of the device is divided, and in active layer 163, photo amplifying region 161 and saturable absorption region 162 are provided.
In semiconductor laser device 160, a current Ih injected to optical amplification region 161 is set to an intermediate value of oscillation threshold values corresponding to voltages VA and VB. Semiconductor laser device 160 receives at saturable absorption region 162 a signal voltage Vh with the forward bias voltage alternately changing to VA and VB, and outputs output light POUT from active layer 163. Semiconductor laser device 160 has the characteristic of bistable state as described with reference to FIG. 14.
The shape of hysteresis shown in FIG. 14 varies in accordance with current injection or voltage application to saturable absorption region 162. When a current is injected or forward bias voltage is applied to saturable absorption region 162, carrier concentration increases and, hence, the effect of light absorption decreases. Thus, hysteresis as a whole moves to the side of lower value of injection current, and the value of rising threshold Pth1 lowers. Therefore, in semiconductor laser device 160, the oscillation threshold value can be changed by increasing/decreasing the light absorption effect of saturable absorption region 162.
FIG. 18 illustrates an operation of semiconductor laser device 160 in the bistable state shown in FIG. 17.
Semiconductor laser device 160 includes saturable absorption region 162 that constitutes a loss to the oscillation light in active layer 163. Therefore, when current is injected only to the optical amplification region 116, optical output increases non-linearly at a certain current value, as shown in FIG. 18. Specifically, when the forward bias voltage applied to saturable absorption region 162 increases from VA to VB (VA<VB), carriers in saturable absorption region 162 increases and optical loss decreases accordingly. As a result, the current value (rising threshold value) at which the optical output increases lowers from IhA to IhB.
Therefore, by applying forward bias signal voltage Vh, of which voltage changes from VA to VB, to saturable absorption region 162 while a constant bias current Ih (IhB<Ih<IhA) is injected to optical amplification region 161, the rising threshold comes to fluctuate between IhA and IhB, whereby a modulated output light Pout can be obtained.
With the detection sensitivity of a conventional photodiode, it is impossible to detect a weak signal that has been attenuated due to the loss along the transmission path of ultra-high-speed communication.
Though a light signal receiving apparatus using a histable semiconductor laser can amplify a weak light signal around oscillation threshold value, its performance is limited that the light signal cannot be amplified unless the intensity of the injection light goes up/down across the threshold value. Therefore, optical amplification by the bistable semiconductor laser is not effective on a signal having small amplitude or a signal of which intensity is attenuated below the threshold value.
The stochastic resonance apparatus in accordance with Japanese Patent Laying-Open No. 2004-214407 provides a high-speed output signal having amplified intensity and well-shaped waveform emphasizing the period of the input signal, by using a bistable semiconductor laser exhibiting hysteresis in the input-output characteristic. In such a stochastic resonance apparatus, it is important to optimally determine and control the shape of hysteresis in accordance with the input signal, while sufficient description in this connection cannot be found in Japanese Patent Laying-Open No. 2004-214407.
According to the method of driving with forward bias voltage of bistable semiconductor laser described in Japanese Patent Laying-Open No. 02-137383, the light signal can be amplified by a simple driving method. In the saturable absorption region having light absorbing effect, however, a depletion layer serves as a junction capacitance, and dominates the speed of response based on CR time constant. Therefore, driven with forward bias voltage, the speed of response becomes slower. As a result, pattern effect tends to be induced, resulting in unsatisfactory reproduction of the signal.